1. Field of the Invention
This invention relates to an acoustic diaphragm, a method for producing the acoustic diaphragm and an improved method for filling a paper diaphragm.
2. Description of the Prior Art
Materials other than paper, such as high polymer material, metals or ceramics, are coming into use as an acoustic diaphragm material. However, the paper diaphragm produced by a paper making technique from cellulose fibers still accounts for a major portion of the currently employed acoustic diaphragms, because the paper diaphragm has such advantages that it may be produced easily, exhibits moderate internal losses and may successfully cope with a wide variety of sound quality requirements because it meets a wide variety of factors governing the sound quality, such as pulp types, freeness or fillers.
Meanwhile, since paper diaphragms are prepared by the paper making technique, it is inevitable that these occupy a more or less large space while exhibiting certain air permeability. Thus, for assuring air tightness of the paper diaphragm, a processing operation called filling is carried out.
Conventional filling operations include laminating a film composed mainly of a high polymer material and exhibiting air tightness on the paper diaphragm surface, or applying a coating composed of a high polymer material dissolved or emulsified in an organic solvent on the paper diaphragm.
However, these filling operations detract from the merits related to the paper diaphragms, namely low density, high toughness and low losses. Besides, since the paper diaphragm material is mixed with heterogeneous materials, the high sound quality proper to the paper diaphragm tends to be affected adversely.
For overcoming these disadvantages, it may be contemplated to prepare paper diaphragms by a so-called mixed paper making technique by adding highly beaten pulp or microfibrilated cellulose during preparation of the paper diaphragm.
With this method, heterogenous materials are not mixed, while the sound quality is not affected significantly.
However, since this method is based essentially on the paper making technique, it is difficult to overcome the problem of air permeability of the diaphragm completely. On the other hand, with the method by mixed paper making, the diaphragm is increased in density, although the sound quality associated with the paper diaphragm is not lost.